parallels: add storage driver

parallels: add storage driver

Parallels Cloud Server has one serious discrepancy with libvirt:
libvirt stores domain configuration files in one place, and storage
files in other places (with the API of storage pools and storage volumes).
Parallels Cloud Server stores all domain data in a single directory,
for example, you may have domain with name fedora-15, which will be
located in '/var/parallels/fedora-15.pvm', and it's hard disk image will be
in '/var/parallels/fedora-15.pvm/harddisk1.hdd'.

I've decided to create storage driver, which produces pseudo-volumes
(xml files with volume description), and they will be 'converted' to
real disk images after attaching to a VM.

So if someone creates VM with one hard disk using virt-manager,
at first virt-manager creates a new volume, and then defines a
domain. We can lookup a volume by path in XML domain definition
and find out location of new domain and size of its hard disk.

Signed-off-by: Dmitry Guryanov <dguryanov@parallels.com>